A concrete structure represented by a bridge and an elevated bridge (hereinafter, simply called a “structure”) is influenced by a weather change, ground deformation, and load bearing, in addition to its own deterioration over the years. When they are collected and bad conditions overlap, the structure may partially break down or peel, for example, possibly leading to a damage to or an accident on a third party. Accordingly, the structure is required to be inspected and observed constantly in order to prevent occurrence of flaking of the structure.
As a method for inspection and observation of the structure, there has been studied a method of infrared inspection which is capable of inspecting the structure in a wide range with high efficiency without necessity of approaching the structure. FIG. 13 shows the time course of changes in three temperatures, namely an atmosphere temperature, a surface temperature of a sound portion of the structure, and a surface temperature of an abnormal portion of the structure. As shown in FIG. 13, a difference is produced between the surface temperature of the abnormal portion and the surface temperature of the sound portion with a change of the heat environment of the structure and the heat environment around it (the atmosphere temperature change here).
FIGS. 14A and 14B show a concept of a phenomenon that a temperature difference is produced. FIG. 14A shows a state of daytime, and FIG. 14B shows a state of nighttime. As shown in FIG. 14A, the atmosphere temperature is higher than the temperature of a concrete 80 in a time zone t1 of FIG. 13, and heat is conducted from outside to inside of the concrete 80. Since the heat conduction is blocked at an abnormal portion 81, the heat conducted from the outside of the concrete 80 remains in a surface portion 82 between the concrete surface and the abnormal portion. As a result, the temperature of a front surface 83 of the surface portion 82 where the abnormal portion 81 is present (this is called “the surface temperature of the abnormal portion 81”) becomes higher than the surface temperature around it. As shown in FIG. 14B, the atmosphere temperature is lower than the temperature of the concrete 80 in a time zone t2 of FIG. 13, and heat is conducted from inside to outside of the concrete 80. Since heat conduction is blocked at the abnormal portion 81, the heat conducted from the inside of the concrete 80 is not conducted to the surface portion 82 which is between the concrete surface and the abnormal portion 81. As a result, the surface temperature of the abnormal portion 81 becomes lower than the surface temperature around it. The method of infrared inspection uses the above phenomenon, and measures the surface temperature of the structure by the infrared camera and discriminates between the sound portion and the abnormal portion by using the temperature difference.
The method of infrared inspection can find the abnormal portion when the temperature difference between the sound portion and the abnormal portion is large to some extent but cannot find the abnormal portion when the temperature difference between the sound portion and the abnormal portion is small. Therefore, it is important to perform the infrared inspection in the heat environment where there is a temperature difference between the sound portion and the abnormal portion. Conventionally, the following methods are used to find such a heat environment.
(1) Method for Estimation of Heat Environment by Atmosphere Temperature Observation
This method estimates whether or not the environment is suitable for infrared inspection by measuring the atmosphere temperature around the structure to be measured.
(2) Heat Environment Assessment Method Using Test Specimen
As shown in FIG. 15, this method assesses whether or not the heat environment of a structure 90 is suitable for infrared inspection by disposing in the vicinity of the structure 90 concrete blocks which are artificially and partly formed with an abnormal portion to represent the structure 90 to be measured, namely test specimens 91, 92 and 93, taking pictures of the test specimens 91, 92 and 93 by an infrared camera, and checking the abnormal portion on the photographed infrared thermal images. The test specimen 91 is disposed just below the structure, the test specimen 92 is disposed at a position which is in the sun during an afternoon time zone, and the test specimen 93 is disposed at a position which is in the sun during a morning time zone. The test specimen 91 is a concrete block placed in an instrument shelter, but the test specimens 92 and 93 are concrete blocks exposed. This type of method is disclosed in, for example, the following Patent References 1 and 2.    Patent Reference 1: JP 2005-140622 A    Patent Reference 2: JP 2006-329760 A